Dayside Magnetosphere Interactions E-Book

• Herausgeber: John Wiley & Sons
• Kategorie: Wissenschaft und neue Technologien
• Serie: Geophysical Monograph Series
• Sprache: Englisch

Seitenzahl: 896

Veröffentlichungsjahr: 2020

Beschreibung

Exploring the processes and phenomena of Earth's dayside magnetosphere Energy and momentum transfer, initially taking place at the dayside magnetopause, is responsible for a variety of phenomenon that we can measure on the ground. Data obtained from observations of Earth's dayside magnetosphere increases our knowledge of the processes by which solar wind mass, momentum, and energy enter the magnetosphere. Dayside Magnetosphere Interactions outlines the physics and processes of dayside magnetospheric phenomena, the role of solar wind in generating ultra-low frequency waves, and solar wind-magnetosphere-ionosphere coupling. Volume highlights include: * Phenomena across different temporal and spatial scales * Discussions on dayside aurora, plume dynamics, and related dayside reconnection * Results from spacecraft observations, ground-based observations, and simulations * Discoveries from the Magnetospheric Multiscale Mission and Van Allen Probes era * Exploration of foreshock, bow shock, magnetosheath, magnetopause, and cusps * Examination of similar processes occurring around other planets The American Geophysical Union promotes discovery in Earth and space science for the benefit of humanity. Its publications disseminate scientific knowledge and provide resources for researchers, students, and professionals. Find out more about this book from this Q&A with the editors

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Geophysical Monograph 248

Dayside Magnetosphere Interactions

Qiugang ZongPhilippe EscoubetDavid SibeckGuan LeHui Zhang

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CONTRIBUTORS

V. AngelopoulosDepartment of Earth, Planetary and Space Sciences, University of California, Los Angeles, CA, USA

J. BortnikDepartment of Atmospheric and Oceanic Sciences, University of California, Los Angeles, CA, USA

X. H. ChenSchool of Space and Environment, Beihang University, Beijing, China

L. B. ClausenDepartment of Physics, University of Oslo, Oslo, Norway

A. J. CosterMassachusetts Institute of Technology Haystack Observatory, Westford, MA, USA

A. W. DegelingShandong Provincial Key Laboratory of Optical Astronomy and Solar‐Terrestrial Environment, School of Space Science and Physics, Institute of Space Sciences, Shandong University, Weihai, Shandong, China

E. F. DonovanDepartment of Physics and Astronomy, University of Calgary, Calgary, Alberta, Canada

P. J. EricksonMassachusetts Institute of Technology Haystack Observatory, Westford, MA, USA

Philippe EscoubetESA European Space Research and Technology Centre, Noordwijk, The Netherlands

Mei‐Ching FokGeospace Physics Laboratory, NASA Goddard Space Flight Center, Greenbelt, MD, USA

J. C. FosterMassachusetts Institute of Technology Haystack Observatory, Westford, MA, USA

H. S. FuSchool of Space and Environment, Beihang University, Beijing, China

S. Y. FuInstitute of Space Physics and Applied Technology, School of Earth and Space Sciences, Peking University, Beijing, China

M. L. GoldsteinSpace Science Institute and Goddard Space Flight Center, Greenbelt, MA, USA

De‐Sheng HanState Key Laboratory of Marine Geology, School of Ocean and Earth Science, Tongji University, Shanghai, China

J. S. HeInstitute of Space Physics and Applied Technology, School of Earth and Space Sciences, Peking University, Beijing, China

D. A. KozlovInstitute of Solar‐Terrestrial Physics SB RAS, Irkutsk, Russia

H. LaaksoESA European Space Astronomy Centre, Madrid, Spain

Guan LeNASA Goddard Space Flight Center, Greenbelt, MD, USA

A. S. LeonovichInstitute of Solar‐Terrestrial Physics SB RAS, Irkutsk, Russia

W. LiCenter for Space Physics, Boston University, Boston, MA, USA

Q. MaDepartment of Atmospheric and Oceanic Sciences, University of California, Los Angeles, CA, USA; and Center for Space Physics, Boston University, Boston, MA, USA

Yu‐Zhang MaShandong Provincial Key Laboratory of Optical Astronomy and Solar‐Terrestrial Environment, School of Space Science and Physics, Institute of Space Sciences, Shandong University, Weihai, Shandong, China

A. MassonESA European Space Astronomy Centre, Madrid, Spain

J. I. MoenDepartment of Physics, University of Oslo, Oslo, Norway

T. NagatsumaNational Institute of Information and Communications Technology, Tokyo, Japan

Z. NěmečekFaculty of Mathematics and Physics, Charles University, Prague, Czech Republic

Y. NishimuraDepartment of Electrical and Computer Engineering and Center for Space Physics, Boston University, Boston, MA, USA; and Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, CA, USA

Katariina NykyriCentre of Space and Atmospheric Research, Department of Physical Sciences, Embry‐Riddle Aeronautical University, Daytona Beach, FL, USA

V. OlshevskyCenter for mathematical Plasma Astrophysics, Department of Mathematics, KU Leuven, Leuven, Belgium

A. OttoGeophysical Institute, University of Alaska Fairbanks, Fairbanks, AK, USA

Z. Y. PuInstitute of Space Physics and Applied Technology, School of Earth and Space Sciences, Peking University, Beijing, China

R. RankinDepartment of Physics, University of Alberta, Edmonton, Alberta, Canada

Jie RenInstitute of Space Physics and Applied Technology, School of Earth and Space Sciences, Peking University, Beijing, China

J. ŠafránkováFaculty of Mathematics and Physics, Charles University, Prague, Czech Republic

X.‐C. ShenShandong Provincial Key Laboratory of Optical Astronomy and Solar‐Terrestrial Environment, School of Space Science and Physics, Institute of Space Sciences, Shandong University, Weihai, Shandong, China; and Center for Space Physics, Boston University, Boston, MA, USA

Q. Q. ShiShandong Provincial Key Laboratory of Optical Astronomy and Solar‐Terrestrial Environment, School of Space Science and Physics, Institute of Space Sciences, Shandong University, Weihai, Shandong, China

David SibeckNASA Goddard Space Flight Center, Greenbelt, MD, USA

J. ŠimůnekInstitute of Atmospheric Physics, Czech Academy of Science, Prague, Czech Republic

D. SydorenkoDepartment of Physics, University of Alberta, Edmonton, Alberta, Canada

R. M. ThorneDepartment of Atmospheric and Oceanic Sciences, University of California, Los Angeles, CA, USA

A. M. TianShandong Provincial Key Laboratory of Optical Astronomy and Solar‐Terrestrial Environment, School of Space Science and Physics, Institute of Space Sciences, Shandong University, Weihai, Shandong, China

A. VaivadsSwedish Institute of Space Physics, Uppsala, Sweden

B. M. WalshDepartment of Electrical and Computer Engineering, Boston University, Boston, MA, USA

B. WangDepartment of Atmospheric and Oceanic Sciences, University of California, Los Angeles, CA, USA; and

Department of Astronomy and Center for Space Physics, Boston University, Boston, MA, USA

C. R. WangDepartment of Physics, University of Alberta, Edmonton, Alberta, Canada

Yong WangShandong Provincial Key Laboratory of Optical Astronomy and Solar‐Terrestrial Environment, School of Space Science and Physics, Institute of Space Sciences, Shandong University, Weihai, Shandong, China

Y. F. WangInstitute of Space Physics and Applied Technology, School of Earth and Space Sciences, Peking University, Beijing, China

Z. WangSchool of Space and Environment, Beihang University, Beijing, China

G. Whittall‐ScherfeeDepartment of Physics, University of Alberta, Edmonton, Alberta, Canada

J. R. WygantDepartment of Physics and Astronomy, University of Minnesota, Minneapolis, MN, USA

Zan‐Yang XingShandong Provincial Key Laboratory of Optical Astronomy and Solar‐Terrestrial Environment, School of Space Science and Physics, Institute of Space Sciences, Shandong University, Weihai, Shandong, China

S. T. YaoShandong Provincial Key Laboratory of Optical Astronomy and Solar‐Terrestrial Environment, School of Space Science and Physics, Institute of Space Sciences, Shandong University, Weihai, Shandong, China

Hui ZhangGeophysical Institute and Physics Department, University of Alaska Fairbanks, Fairbanks, AK, USA

Qing‐He ZhangShandong Provincial Key Laboratory of Optical Astronomy and Solar‐Terrestrial Environment, School of Space Science and Physics, Institute of Space Sciences, Shandong University, Weihai, Shandong, China

H. Y. ZhaoInstitute of Space Physics and Applied Technology, School of Earth and Space Sciences, Peking University, Beijing, China

X. Z. ZhouInstitute of Space Physics and Applied Technology, School of Earth and Space Sciences, Peking University, Beijing, China

Qiugang ZongInstitute of Space Physics and Applied Technology, School of Earth and Space Sciences, Peking University, Beijing, China

Y. ZouDepartment of Astronomy and Center for Space Physics, Boston University, Boston, MA, USA; and Cooperative Programs for the Advancement of Earth System Science, University Corporation for Atmospheric Research, Boulder, CO, USA

PREFACE

Magnetospheric physics addresses a vast array of topics, including the interaction of the solar wind with the magnetosphere, how the magnetosphere interacts with the ionosphere, and a host of processes that occur within the dayside magnetosphere.

The AGU Chapman Conference on Dayside Magnetosphere Interactions held in July 2017 in Chengdu, China, addressed the processes by which solar wind mass, momentum, and energy enter the magnetosphere. Topics discussed included the foreshock, bow shock, magnetosheath, magnetopause, and cusps; the dayside magnetosphere; and both the dayside polar and equatorial ionosphere. The meeting was particularly timely due to the results expected from NASA’s magnetospheric multiscale (MMS) mission that was launched in March 2015, arrays of new ground‐based instrumentation being installed, as well as the ongoing operations of NASA’s Time History of Events and Macroscale Interactions during Substorms (THEMIS) and Van Allen Probes missions, European Space Agency (ESA)’s Cluster mission, and Japan Aerospace Exploration Agency (JAXA)’s Geotail mission. Parallel processes occur at other planets, and recent results from NASA’s Mars Atmosphere and Volatile Evolution (MAVEN) mission to Mars, as well as ESA’s Mars and Venus Express missions were also discussed.

The 2017 Chapman Conference built upon two previous Chapman Conferences on the dayside boundary of the magnetosphere and their related publications: Earth’s Low‐Latitude Boundary Layer (Geophysical Monograph 133, 2003) and Physics of the Magnetopause (Geophysical Monograph 90, 1995).

These two Chapman Conferences on dayside dynamics were held more than one or two solar cycles ago. Thus, a Chapman Conference on dayside interactions was very much overdue given the new data sets brought by the constellation missions launched since then.

This monograph includes papers presented at the 2017 Chapman Conference as well as invited papers from experts who did not attend. It starts with a brief history of dayside magnetospheric physics and transients (Otto, Chapter 1). Part I considers the physics of dayside magnetospheric response to solar wind discontinuities. This section presents a summary by the Geospace Environment Modeling (GEM) Focus Group of findings on transient phenomena at the magnetopause and bow shock, and their geoeffects (Zhang and Zong, Chapter 2), solar wind–magnetosphere–ionosphere interactions driven by foreshock transients, magnetosheath high‐speed jets, and localized magnetopause reconnection (Nishimura et al., Chapter 3), and solar wind dynamic pressure changes (Shi et al., Chapter 5). Throat aurora that might be driven by magnetosheath high‐speed jets is also discussed (Han, Chapter 4). Part II is devoted to the structure and dynamics of dayside boundaries. This section includes Cluster mission’s recent highlights at dayside boundaries (Escoubet et al., Chapter 6), the structure and dynamics of the magnetopause and the magnetosheath (Nykyri, Chapter 7; Němeček et al., Chapter 8), and a review of different methods to find magnetic nulls and reconstruct magnetic field topology (Fu et al., Chapter 9). Part III examines the roles of solar wind sources on wave generations and dynamic processes in the inner magnetosphere. This includes a theoretic study on the spatial structure of toroidal standing Alfvén waves in the magnetosphere (Leonovich and Kozlov, Chapter 10), wave–particle interactions in Earth’s outer radiation belt (Rankin et al., Chapter 11; Li et al., Chapter 12), and a review of the current status of radiation belt and ring current modeling (Fok, Chapter 13). Part IV addresses cold plasmas of the ionospheric origin including the geospace plume (Foster, Chapter 14), ionospheric patches (Zhang et al., Chapter 16), and their interaction with ULF waves in the magnetosphere (Zong et al., Chapters 15 and 17).

Over 128 scientists from more than 20 countries participated in the conference. We acknowledge help from AGU staff for the success of the conference as well as the completion of this monograph. Also we acknowledge financial support from National Science Foundation and Peking University.

Qiugang ZongPeking University, China

Philippe EscoubetESA European Space Research and Technology Centre, The Netherlands

David Sibeck, Guan LeNASA Goddard Space Flight Center, USA

Hui ZhangUniversity of Alaska Fairbanks, USA

1A Brief History of Dayside Magnetospheric Physics

A. Otto

Geophysical Institute, University of Alaska Fairbanks, Fairbanks, AK, USA

ABSTRACT

Dayside magnetospheric physics has an early history that is closely related to our understanding of the magnetosphere as a whole. The early years of magnetospheric physics are somewhat reminiscent of the gold rush era or the exploration of the American west. Moving into the satellite era, our field had, for the first time, the opportunity to examine in‐situ dayside plasma processes to confirm or reject theories, something that neither solar nor astrophysics can do. Since the late 1970s, with better and faster instrumentation, we have been able to develop a detailed understanding of magnetopause and bow shock plasma physics, where transient phenomena play a critical role. This article provides a brief history of these periods of time and how these led into a modern understanding of dayside physics and transient events.

1.1. SETTING THE STAGE: THE PRE‐SATELLITE ERA

At the turn of the nineteenth century, it was known that the Earth's magnetic field could at times undergo strong perturbations that seemed to correlate with auroral activity. It was also hypothesized that these magnetic perturbations were caused by processes on the sun. The most prominent example of this relation was the great flare observed by Richard Carrington on 1 September 1859 (Carrington, 1859) and the geomagnetic response. However, such a connection between solar processes and geomagnetism was met by strong criticism at the time.

In the years around the turn of the nineteenth century, Kristian Birkeland undertook a number of expeditions to the auroral zone. He was the first to identify what he called the polar elementary storm which is now known as the auroral substorm. Birkeland provided a highly detailed description and analysis of his observations and implied the existence of vertical currents in the upper atmosphere as closure for the horizontal currents he inferred from magnetic observations. Based on the observations and his gas discharge “Terella” experiments studying the paths of electrons in a dipole representing Earth, Birkeland was convinced that the aurora and associated magnetic perturbations were caused by precipitating electrons from the sun (Birkeland, 1908). He also provided a reasonable estimate of the electric currents and the power associated with the auroral activity. Some years later, Sydney Chapman, a brilliant mathematician, published his first model for geomagnetic storms (Chapman, 1918a). Although most of this work involved horizontal currents in the upper atmosphere, the batteries for these currents were “vertical motions.” These he assumed to be provided by a mostly single charged particle precipitation of solar origin although he noted that this idea was not well appreciated in the science community (Chapman, 1918b). It was only a year later that Frederick Lindemann pointed out that the supposed solar corpuscular stream cannot be single‐charged and must contain ions and electrons to be charge neutral (Lindemann, 1919).

Based on a charge neutral, ideally conducting solar stream Chapman and Ferraro presented a new theory of magnetic storms where the geomagnetic field is compressed facing the stream and extended in its wake (Chapman & Ferraro, 1931) somewhat similar to our picture of the magnetosphere (Figure 1.1). They called this a magnetic hollow where solar wind particles could access the upper atmosphere only through “two horns” at the location of the cusps of the magnetic field. This model presented for the first time the concept of a magnetopause as the boundary between the solar plasma and the Earth's closed magnetosphere, and this model dominated the view in the science community for decades. The model agreed qualitatively with most magnetic storm properties particularly for the initial increase of the magnetic field (sudden commencement), however, it was not convincing for the main phase magnetic depression. Chapman and Bartels (1940, p. 810) remarked that a more efficient particle entry and energization were needed than provided in the closed magnetic field model. A different model for magnetic storms and plasma entry in the form of clouds was suggested by Hannes Alfvén (1940) that generated an ongoing controversy for two decades (e.g., Alfvén, 1958).

Figure 1.1 Illustration of the “magnetic hollow” (magnetic cavity) exposed to the ideally conducting solar plasma.

Source: From Chapman and Ferraro (1931).

It should be noted that, at the time, the stream of solar plasma was generally assumed to be transient and localized although Biermann (1951) demonstrated through cometary tail observations that the stream of solar material must, in fact, be continuous. However, Chapman shared the view with some in the community of an invisible solar corona that extended beyond Earth's orbit and expanded at a low velocity of a few 10 km s−1 (Parker, 1997). Eugene Parker realized that not both views on the stream of solar plasma could be true, and, almost coincident with the launch of the first satellites, and Parker (1958) published his famous theory of the solar wind and coined the name. Somewhat typical of this time is an episode around this publication (Parker, 1997). Parker had submitted his paper to ApJ where Chandrasekhar was editor. So, Chandrasekhar came to Parker's office one day and told him that all (highly qualified) reviewers regarded the paper as wrong and whether he really wanted to publish it. Parker said “yes,” since the reviewers had no explicit objection to the physical arguments, and after a moment, Chandrasekhar responded “Alright, I will publish it.” Still, 2 years later on an international conference, Chamberlain argued that the supposed supersonic solar wind was the result of a wrong integration constant and the limited heat supply allowed only for a slow expansion of about 20 km s−1 at 1 AU (Chamberlain, 1960, 1961). Fortunately, Parker's work and reputation were saved by the first satellite observations of the solar wind (Bridge et al., 1962; Gringauz et al., 1962; Snyder & Neugebauer, 1963). It should be mentioned, however, that for very rare conditions the solar wind can indeed be almost absent such that Chamberlain's view on the topic was not entirely wrong.

1.2. INTO THE SATELLITE ERA

Similar to the importance of a new understanding of electrodynamics and electricity for progress in the first half of the nineteenth century, plasma physics and particularly the formulation of the magnetohydrodynamic (MHD) equations by Alfvén, Schlüter, and others enabled the theoretical understanding of the newly discovered magnetosphere. Even though there had been and still is criticism for the MHD approach, the rapid progress in the late 1950s and early 1960s is inconceivable without the framework of a magnetofluid description, the work by Parker on the solar wind being an excellent example. This theoretical framework and the new in‐situ satellite measurements that became available since 1958 advanced our knowledge of the dayside bow shock and magnetopause physics rapidly.

Gold (1955) realized that a shock likely propagated in the stream of solar plasma to cause the sudden rapid compression associated with the sudden commencement of magnetic storms. Based on the short duration (few minutes), he also implied that the solar plasma must be magnetized because otherwise the shock width, based on the very large mean free path, would be too large to explain the fast compression. Several years later, the existence of a bow shock in front of the newly discovered magnetosphere had been suggested (Axford, 1962; Gold, 1962; Kellogg, 1962; Zhigulev, 1959). For instance, Ian Axford produced the teardrop shape of the magnetosphere with a bow shock and discussed the stability of the magnetospheric boundary. He also provided the familiar estimate of the magnetopause standoff distance and argued correctly that the magnetic boundary encountered by Pioneers 1 and 5 (Sonett, 1960; Sonett et al., 1960) was the bow shock rather than the magnetopause as had been originally assumed.

Figure 1.2 The “open magnetosphere” as suggested with x lines on the day and night side.

Source: From Dungey (1961).

In the following years, properties of the bow shock such as shape, motion, and upstream particle acceleration were examined based on the newly available observations. Burlaga and Ogilvie (1968) carried out a detailed comparison of Explorer 34 observations with theoretical shock predictions and found good agreement. Some diffuse shock encounters were interpreted as the shock moving. Models using hydrodynamic flow around a model obstacle were employed to make predictions on the shape of the bow shock (Spreiter & Jones, 1963; Spreiter et al., 1966), and satellite observations of bow shock locations provided empirical models of the bow shock shape and distance that agreed well with hydrodynamic predictions (Fairfield, 1967; Fairfield & Ness, 1967; Gosling et al., 1967). Also, at the time, satellites provided the first evidence of upstream moving electrons (Fan et al., 1966) and ions (Asbridge et al., 1968) upstream of the bow shock. Satellite observations also made it apparent that these were regions of increased wave turbulence (Fairfield, 1969). The foreshock regions were a surprise because they had not been predicted by theory demonstrating the kinetic character of the shock structure and showing limitations of the MHD framework. Sonnerup (1969) offered a simple explanation of the upstream acceleration in terms of a displacement of particles along the interplanetary electric field during the reflection process, and Greenstadt (1976) discussed the geometry and energy distribution of the reflected particles, both of which seemed to agree reasonably well with observations (Paschmann et al., 1980). Early indications of transient structure or events at the bow shock were identified in Vela 3 observations by Greenstadt et al. (1968).

This understanding of the bow shock was important for the shape of the magnetosphere but did not offer directly an explanation for the entry of solar wind particles into the magnetosphere and the causes of the aurora and magnetic perturbations during geomagnetic activity. Motivated by solar magnetic field eruptions, Sweet (1958) and Parker (1957) derived the first model of magnetic reconnection based on magnetic neutral lines, at the time called magnetic field annihilation or magnetic field merging. Jim Dungey, who was familiar with this work and with auroral observations, became convinced that some auroral boundaries were so thin that they should be topological boundaries. Therefore, he postulated two neutral (x) lines for the magnetosphere (Figure 1.2), which meant that the magnetosphere should, in fact, be open (Dungey, 1961). Dungey was well aware that the associated electric field in his model would cycle closed geomagnetic flux into open flux on the dayside and vice versa in the tail. However, Sweet and Parker reconnection was far too slow in a highly collisionless plasma because it depended on slow magnetic diffusion in stretched thin current sheet. Arthur Kantrowitz suggested to Harry Petschek that waves might be important for this problem, and Petschek realized that the dissipation does not have to occur in a thin sheet along the x line but can happen all along the boundaries of the reconnection outflow region in the form of shocks. Therefore, Petschek's (1964) reconnection model depended only weakly on the actual dissipation at the x line and reconnection was much faster with a normalized rate of Order 0.1 which agrees well with most current observations of magnetic reconnection. Petschek presented his theory at a solar flare conference and noteworthy is the comment by Sweet: “Dr. Parker and I have been living with this problem for several years… Your solution struck me at once as the solution for which we have been seeking.” Dungey's and Petschek's work, however, also opened a new controversy as to whether the magnetosphere was open or closed based on Chapman's work. Note that Petschek's basic consideration is still valid for fast reconnection, although the physics of the outflow region is more complicated and depends on geometry and kinetic processes.

Related to reconnection, it should be mentioned that Furth et al. (1963) developed the theory of the resistive tearing mode as the linear instability leading to reconnection, and Coppi et al. (1966) applied the collisionless tearing mode for the first time to explain the onset of reconnection in the magnetotail.

In order to explain high‐latitude phenomena like aurora, magnetic perturbations, and polar cap convection, Axford and Hines (1961) suggested viscous interaction at the magnetospheric flank boundaries (Figure 1.3). They were unspecific other than mentioning some instability or eddy viscosity that would provide the viscous coupling, and, in fact, Dungey had considered viscosity in a different context. Both Axford and Parker had speculated about Kelvin–Helmholtz (KH) waves at the magnetospheric boundary.

Figure 1.3 Sketch illustrating viscous momentum transfer from the solar wind and the resulting magnetospheric convection in the equatorial plane.

Source: From Axford and Hines (1961).

Although various studies looked at implications of reconnection and viscous interaction, such as global flux transport, and the response of the convection and tail dynamics based on the interplanetary magnetic field (IMF) orientation, an explicit confirmation from satellite observations would not become available for almost another 20 years. In fact, until the late 1970s, in‐situ observations had no clear identification of low‐latitude dayside reconnection (Fairfield, 1979; Haerendel et al., 1978) or at best indicated that reconnection might occur (Sonnerup & Ledley, 1979).

There was still important work on reconnection such as the suggestion of multiple dayside reconnection patches (Nishida & Maezawa, 1971), models of steady reconnection (Vasyliunas, 1975), and indirect evidence such as cusp latitude control of reconnection (Burch, 1973). However, the lack of actual in‐situ signatures also generated increasing skepticism (Heikkila, 1975) and alternative models for the plasma entry termed “impulsive penetration” (Lemaire, 1977).

1.3. INTO A MATURE FIELD: DAYSIDE TRANSIENT PROCESSES

In October 1977, International Sun‐Earth Explorer (ISEE) 1 and 2 were launched to study the solar wind‐magnetosphere interaction, and in August 1984, the Active Magnetospheric Particle Tracer Explorers (AMPTE) satellites were launched to study access of solar wind ions to the magnetosphere and to provide two‐point observations. Compared to prior missions, the spacecraft had superior instrumentation providing better and much higher time resolution data. Immediately after the ISEE launch, two different signatures of reconnection have been identified. Russell and Elphic (1978) saw the frequent occurrence of strong dipolar perturbations of the magnetic field close to the magnetopause and interpreted this as the signature of magnetic flux ropes that swept past the satellite, connecting the magnetosphere with the magnetosheath (Figure 1.4). They proposed that these flux ropes were generated by a patch of magnetopause reconnection and termed these events magnetic flux transfer events (FTEs). Quite different from this signature, Paschmann et al. (1979) reported strong plasma acceleration tangential to the magnetopause that satisfied the conditions of steady reconnection as formulated by Petschek and other reconnection models for more general current sheet geometries.

It was almost as if a levee had broken. After the initial publications there was a flurry of excellent work that identified in‐situ signatures and properties of both, steady reconnection (Eastman & Frank, 1982; Gosling et al., 1982; Scholer et al., 1981; Sonnerup et al., 1981) and FTEs (Daly et al., 1984; Daly & Keppler, 1982; Paschmann et al., 1982; Russell & Elphic, 1979) in much detail. In 1984, this evolution culminated in a Geophysical Monograph on Magnetic Reconnection (Hones, 1984) but the flood of exciting studies on the topic has continued unbroken until today.

Figure 1.4 Sketch of the proposed magnetic flux rope connecting the magnetosheath and magnetosphere which causes the dipolar magnetic field signature for flux transfer events (FTEs).

Source: From Russell and Elphic (1978).

Following the observations of these quite different reconnection signatures, there has been a debate on how FTEs were formed, and whether FTEs and signatures of fast flows represented different modes of magnetopause reconnection. This discussion was also stimulated by another technological advance, that is the development of sophisticated two‐ and three‐dimensional computer simulations. In part based on these, several models for FTE formation were proposed, that is, impulsive single x‐line reconnection (Scholer, 1988), multiple x‐line reconnection (Lee & Fu, 1985), reconnection in localized single patches (Otto, 1990; Russell & Elphic, 1978), or multiple patches (Nishida, 1989; Otto, 1995) distributed over the magnetopause. However, it is likely fair to say that the type of reconnection signature depends on where or how the reconnected flux geometry is encountered. For some signatures attributed to FTEs, it can also not be excluded that they may be caused by transient solar wind pressure variations (Otto, 1995; Sibeck, 1990).

Computer simulations also contributed much to another transient process that is the viscous transport of momentum through KH waves (Miura, 1984) and the transport of mass through reconnection within KH waves for north‐ and southward IMF conditions (Ma et al., 2014; Nykyri & Otto, 2001; Otto, 2006; Otto & Fairfield, 2000).

Better instrumentation and higher temporal resolution also enabled the discovery and investigation of transient bow shock events. Steven Schwartz reported the observation of highly unexpected events termed “active current sheets” upstream of the Earth's bow shock (Schwartz et al., 1985), which are now termed hot flow anomalies (HFAs) (example in Figure 1.5). Soon afterwards, Michelle Thomson reported similar observations and used the term “hot diamagnetic cavities” (Thomsen et al., 1986) for the events characterized by a large increase in temperature, depletion of the magnetic field, and strong deceleration and deflection of the solar wind velocity. Although the foreshock regions were reasonably explored and understood at the time, these events were a mystery because they were large scale, and common understanding was that no information could travel upstream of a fast shock except for kinetic processes. The initial reports were again followed by quite a number of further observations that showed that the events were often at the transition between the quasiparallel and quasi‐perpendicular shocks (Thomsen et al., 1988), did not necessarily have a strong magnetic field depletion at their core (Paschmann et al., 1988) which was often flanked by fast shocks (Fuselier et al., 1987), and had magnetosheath manifestations (Schwartz et al., 1988) indicating that they may have been the result of a disruption and reformation of the bow shock.

In the late 1980s, I frequently visited the Max–Planck Institute in Garching for a collaboration and remember quite well some intriguing discussions with Götz Paschmann on these extraordinary events. We speculated whether reconnection could provide the observed enormous change of momentum because the events seemed to be associated with tangential discontinuities. An answer was provided by hybrid simulations demonstrating that the interaction of a discontinuity with a fast shock can indeed generate structures like the observed HFAs, provided the motional electric field has the correct sign. Later these results were confirmed by global hybrid simulations (Lin, 1997; Omidi & Sibeck, 2007).